A shock absorber is disclosed respectively by US2005/0011712A1 published by the United States Patent and Trademarks Office in 2005, JPH07/019,642U1 published by the Japan Patent Office in 1995 and JP2000-356237A1 published by the Japan Patent Office in 2000. Each of these shock absorbers comprises a cylinder, a main piston partitioning the cylinder into two operating chambers, a passage provided in the main piston and connecting with the two operating chambers and two pressure chambers divided by a free piston and connected respectively with the two operating chambers.
These shock absorbers produce a relatively small damping force as a result of small-amplitude vibration due to the displacement of working oil between the pressure chambers and the operating chambers in response to the displacement of the free piston. Conversely large-amplitude vibrations result in the free piston displacing to the end of a stroke and thus working oil can not move between each operating chamber and the corresponding pressure chamber. In this case, the working oil moves directly between the two operating chambers through the passage provided through the main piston. The provision of a resistance element such as a leaf valve in the passage produces a large damping force.
Thus the shock absorbers rapidly increase the damping force when the free piston reaches the end of the stroke. The prior art shock absorbers provide a cushion projecting in the direction of displacement of the free piston in order to mitigate a sharp variation in the damping force. When the free piston has almost reached the end of the stroke, the cushion abuts with the wall of the pressure chamber. Consequently the displacement of the free piston is gradually impeded and the damping force undergoes a gradual increase.